使用python中的java库（java包的Python包装器）

Question

I have a java library in jar form which can be used to extract data from files(excel,xml etc). As its in java, it can be used only in java applications. But i need the same library to be used for python projects as well. I have tried py4j etc which takes the objects from jvm. But the library is not an executable and wont be 'run'. I have checked Jython but i need the library to be accessible from Python projects. I have thought about using automated java to python translators, but i would take that as the last resort.

Please suggest some way i can accomplish this.

Answer 1

You can make a one class java program with a thread never ending until you send from Python a notification to do so.

This way the lib would be kept in memory and accessible from your Python program.

This class could be like this (add your needed lib import/init) :

public class TestPy {

    private Thread thread;

    public void die() {
        synchronized (thread) {
            thread.interrupt();
        }    
    }

    public TestPy() {
        thread = new Thread(){
            public void run() {
                try {
                    while (!Thread.interrupted()) {
                        Thread.sleep(500);
                    }
                } catch (InterruptedException e) {
                    Thread.currentThread().interrupt();
                }
            }
        };
        thread.start();    
    }

    public static void main(String[] args) {
        TestPy tp = new TestPy();
        GatewayServer server = new GatewayServer(tp);
        server.start();
    }
}

You would have to launch the java program, use the lib, and then use the die() method to kill the java program in Python :

gateway = JavaGateway()
do your stuff here using the lib
tp = gateway.entry_point
tp.die()

Answer 2

You can write a simple command line Java program which calls the library and saves the results in a format you can read in Python, then you can call the program from Python using os.system .

Another option is to find Python libraries with equivalent functionality to the Java library: you can read excel, xml and other files in Python, that's not a problem.

Answer 3

I haven't learned how to create new instances of java class in a jar file by java constructors, but accidentally found that it's very easy to use any java static methods to access java objects in py2j.

step 1: download py4j zip file from https://pypi.python.org/pypi/py4j . "py4j0.10.0.jar" is in the zip file.

step 2: install py4j by

pip install 'D:\Downloads\py4j-0.10.0.zip'

step 3: add py4j0.10.0.jar as well as the_lib_you_use.jar (like owlapi-distribution-3.5.0.jar for the example below) to build path in your eclipse project

step 4: create AdditionApplication.java, and copy and paste the code of AdditionApplication.java at https://www.py4j.org/ , and run Java application AdditionApplication.java

step 5: after running AdditoinApplication.java, test the example code in a python file:

if __name__ == '__main__':
    pass

from py4j.java_gateway import JavaGateway
gateway = JavaGateway()                   # connect to the JVM

random = gateway.jvm.java.util.Random()   # create a java.util.Random instance
number1 = random.nextInt(10)              # call the Random.nextInt method
number2 = random.nextInt(10)
print(number1,number2)
(2, 7)
addition_app = gateway.entry_point        # get the AdditionApplication instance
addition_app.addition(number1,number2)    # call the addition method


Math = gateway.jvm.java.lang.Math
a = Math.max(4, 6);
print a

IRI = gateway.jvm.org.semanticweb.owlapi.model.IRI

abcIRI = IRI.create('fewf#fe')
print 'abcIRi = ' + str(abcIRI)
print 'abcIRI.getFragment() = ' + abcIRI.getFragment()

The result on console is :

(5, 0)
6
abcIRi = fewf#fe
abcIRI.getFragment() = fe